A polarizing plate, which is one component of the optical device, has a structure in which transparent protective films are laminated to both sides of a polyvinyl alcohol base polarizing film in which iodine or a dye is adsorbed and oriented or a polyvinylene base polarizing film obtained by the dehydration of polyvinyl alcohol. An adhesive has been used in laminating these layers together. Various adhesives have been known for this purpose, examples of which include epoxy, acrylic, and urethane-base types, but there is a drawback in these adhesives generally not to have enough adhesion to stick to a polarizing film. Vinyl acetate adhesives and polyvinyl alcohol adhesives have relatively good adhesion to a polarizing film, but these adhesives have a problem that the adhesive strength is affected by humidity.
In view of this, surface treatments have been performed on a protective film to improve the adhesion between an adhesive agent and a protective film or polarizing film. In a case of using a triacetyl cellulose film, which is commonly employed as a protective film, a method has been utilized in which the triacetyl cellulose film is immersed in a high-temperature and high-concentration alkali solution to be saponified, which renders the surface hydrophilic, and the surface is then coated with an adhesive and stuck together with a polarizing film. Another method has been proposed in which the surface of a cellulose ester film, polycarbonate film, polyester film, or polyacrylic film is treated with a plasma, after which it is stuck together with a polarizing film with an adhesive layer interposed therebetween (Japanese Laid-Open Patent Application 2000-356714).
Also, in recent years liquid crystal display devices have sometimes become to be used under conditions of high temperature and humidity in outdoor or automotive applications, and this creates a need for a polarizing plate with high resistance to heat and moisture and with a high degree of polarization and high transmission for high-resolution color display. To meet this need, there has been proposed a method to use a triacetyl cellulose film as a protective film on one side, and a polycarbonate resin film and/or a polymethyl methacrylate resin film as the other side protective film (Japanese Laid-Open Patent Application H7-56017).
However, there is a problem in both of the above methods that an adhesive layer is interposed in the lamination of the polarizing film and the protective films, making it difficult to reduce a film thickness, to reduce costs and to simplify the production process.
Further, issues with the viewing angle, coloring, and so forth are encountered with liquid crystal display devices obtained by bonding the above-mentioned polarizing plate to a liquid crystal cell. These are due to the birefringence of the liquid crystal molecules in the liquid crystal cell. Therefore, the birefringence of the liquid crystal molecules can be compensated for by disposing a retardation plate having a birefringence between the polarizing plate and the liquid crystal cell, and this can solve the above problems. Nevertheless, an adhesive layer is generally interposed in a lamination of the polarizing plate and the retardation plate, and this prevents further reduction of the laminate film thickness in a liquid crystal display device.